Barrier Properties of Biodegradable Bacterial Polyester Films - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 70

2006)) exhibit the following tendencies. There was no real significant difference in
water sorption behavior between the PHBV copolymers. It can be stated that water
sorption is practically independent of the hydroxyvalerate unit contents in the compo-
sition range studied in literature. The sorption results were close in magnitude of those
of PLLA, PLA, and PCL polymers, for example. We can observe a slight reduction in
water concentration compared with PET and PLA results. Of course, the water con-
centration of PLA, PCL, and PHBV polymers were larger than that of polypropylene
since polyesters contain ester groups able to interact with water molecules. Indeed,
these water sorption uptakes are the consequence of the affinity between the hydroxyl
groups of the water molecules and the ester groups of the backbone even if the ob-
served values are quite low. The small hydrophilic nature and the semi-crystalline
structure are primarily the reason for this water like behavior. Compared to PET fiber
or PLA polymers, the water sorption capacity of PHBV polymers is slightly reduced
due to a higher crystallinity degree.
Although, PHB and PHBV polymers are generally considered as hydrophobic
polymers according to Iordanskii and co-workers (Iordanskii, 1999), it seems that
bacterial polymers slightly interacts with water molecules explaining the water sorbed
concentration inside the film. This hydrophilic character is a function of the ratio be-
tween dispersive interactions (hydrophobic effect) and hydrophilic interactions (polar
and electrostatic effects) and depends on the morphology and degree of orientation of
the anisotropic units inside film. The presence of imperfect crystallites inside polymer
may result in the formation of sites (essentially accessible carbonyl moieties) for the
absorption of water molecules.
At higher water activities, the large water sorption uptakes are usually ascribed to
the clustering of the water molecules inside the polymer to form aggregates because of
the predominance of water-water hydrogen interactions over water-polymer hydrogen
interactions.
The dependence of the ambient humidity on PHBV polymers structure, even if the
impact is small, can dramatically affect the thermal and mechanical properties. This
changing behavior is commonly observed for natural polymers such as cellulose and
starch, and for natural polyesters such as PLA and PCL.
The CO 2 sorption was investigated gravimetrically and the behavior proved to be
linear for all PHBV polymers studied according to Miguel and co-workers (Miguel,
1999b). The low sorption observed indicated that the PHBV polymers have good barrier
properties mainly due to the high degree of crystallinity in this family of bio-resource
based polymers. The authors mentioned a certain susceptibility of these polymers with
respect to CO 2 reflecting some kind of structural rearrangement induced by diffus-
ing molecules. In this case too, the values were comparable with those of common
polymers.
The CO 2 and water sorptions induce similar trend for PHBV polymers due to the
affinity of the backbone structure and the crystallinity degree exhibited. A major chal-
lenge for the material manufacturing concerns the resistance to moisture conditions.
However, when comparing the water vapor resistance of various PHBV polymers to
materials based on mineral oil, it becomes clear that it is possible to produce bio-based